How to know if the model can smile or talk
Ideally, the lips need to be not fused together for a face rig to work properly on the mouth. Otherwise, you'll need to separate the mesh yourself, which can be a pain. Most commercial rigs will not have this problem. Also, if the mouth, teeth, and eyes are not separate objects, they'll be very easy to separate if they have their own vertex groups. Failing that, you can select and isolate by material in the Edit menu. If these things are not stipulated in the details for the figure you're downloading, then you may need to ask the creator specifically.